H7N9 bird flu virus prefers to infect birds to humans

20 June 2013

New research from the MRC's National Institute of Medical
Research (now part of the Francis Crick
Institute) reveals that the current H7N9 bird flu
virus circulating in China has not acquired a preference for human
hosts over birds.

Since the first case of H7N9 infection in humans was reported in
February 2013, there have been 132 confirmed cases and 37 deaths,
all in, or acquired in, mainland China. There is no evidence of
sustained transmission between people and all of the infections
seem to have come from infected poultry.

Flu viruses are categorised into types A, B and C.
Type A viruses are categorised depending on two proteins on their
surface - called haemagglutinin (H) and neuraminidase (N). There
are many different combinations of H and N, such as H7N9 avian
influenza - commonly known as a type of bird flu.

The haemagglutinin is responsible for the virus
binding to host cells through its interactions with a cell receptor
and, subsequently, the delivery of the virus genome into the cell.
Individual binding sites on haemagglutinin bind host cell receptors
weakly, but multiple interactions can result in a very high binding
strength when the virus binds to cells carrying preferred receptor.
This is what determines its affinity for a particular host
species.

Led by Steve Gamblin and John McCauley, the NIMR team studied
how strongly the H7N9 avian virus isolated from infected humans
bound to models of human and bird receptors.

Their results showed that, although the human H7N9 virus had
significantly higher affinity for the human receptor than an H7
haemagglutinin from a typical bird flu virus, it retained stronger
binding to the bird receptor, as is characteristic of bird viruses.
In comparison, past pandemic flu viruses have shown stronger
binding preference for human than bird
receptors.

However the sequence of the haemagglutinin gene of the H7N9
virus isolated from both humans and birds had features that
suggested that it interacts with its receptors in a way that is
unusual for a bird flu virus.

Dr McCauley, Director of NIMR's WHO Influenza Centre, explained:
"Our results indicate that the human H7N9 virus has acquired some
of the receptor-binding characteristics that are typical of
pandemic viruses, but that it retains a preference for avian
receptors. This suggests that efficient transmission of the virus
between humans might be restricted.

"The increase in the number of human cases of H7N9 in China has
stopped and surveillance of poultry markets has revealed only a
small number of affected premises. H7N9 seems to be under control
at present. Nevertheless, H7N9 viruses still pose a pandemic
threat."

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